Hot-melt adhesives are thermoplastic materials that can be heated to a melt and then applied to various substrates. A bond is formed upon cooling and resolidification. Among the most widely used thermoplastic polymers in hot-melt adhesives is ethylene-vinyl acetate copolymer (“EVA”) which is combined with a variety of plasticizers, tackifiers, antioxidants, waxes, and extenders for purposes of improving and/or controlling the viscosity, adhesive properties, shelf-life, stability and cost. Plasticizers have typically included such compounds as polybutenes and phthalates, tackifiers have typically included such compositions as rosin esters and hydrocarbon resins, antioxidants are frequently based upon the known hindered phenol compounds, and wax helps to reduce the melt viscosity in addition to reducing costs.
These hot-melt adhesives have the drawback of often becoming brittle below the glass-transition temperature. Historically, ethylene based semi-crystalline polymers like polyethylene and ethylene vinyl acetate (EVA), have been used in various adhesive applications; however, such polymers have many problems in their end use applications. For example, semi-crystalline linear low density polyethylene (LLDPE) can be used in hot melt adhesive applications where the crystalline network formed on cooling makes a good adhesive free of tack, but the high level of crystallinity causes the material to be brittle. For this reason other monomers, such as vinyl acetate (VA), or alpha-olefins are often co-polymerized with ethylene to break up some of the crystallinity and soften the adhesive. Thus the use of hot-melt adhesives based upon EVA is limited when low temperature conditions of use are desired.
Styrene block copolymers (“SBC”) are independently known as an important class of base polymers for adhesive compositions, particularly for such uses as in hot melt pressure sensitive adhesives in tapes, label stock, diaper assembly and the like. However, because of higher melt viscosities than EVA based compositions, SBC based adhesive compositions are not typically used for packaging where high-speed application is economically desirable.
Certain adhesive composition blends of SBC and EVA are known, even though the base polymers are largely incompatible, in the sense of not being able to form stable blends largely free of separation or stratification and resulting nonuniformity of properties. U.S. Pat. No. 4,345,349 describes book-binding hot-melt adhesive compositions prepared from 15-30% SBC, 5-10% EVA, 25-40% rosin ester tackifier, 25-35% wax diluent and 0.5-3% of a stabilizer, e.g., hindered phenol compound. The ratio of SBC to the ethylene vinyl acetate copolymer is from 1.75/1 to 6/1. The low-temperature flexibility improves by increasing the amount of SBC in the composition and using a high softening point tackifier or high melting point wax shortens setting speed. Setting time, in order to be useful in the described bookbinding process, is to be within 30 seconds, and times within 26 seconds are exemplified. U.S. Pat. No. 4,394,915 describes a hot melt adhesive particularly suitable for polyethylene-terepthalate bottle assemblies comprising typically 20-40% SBC, 5-20% EVA, 30-60% tackifying resin, 10-30% wax or oil, and 0.1-4% stabilizer. The tackifying resin can be any of a number of available rosins or resins, including the aliphatic petroleum resins, but is preferably a polymerized tall oil rosin.
PCT/US97/04161 teaches the use of ethylene based copolymers as hot melt adhesive and these materials are useful in some applications, but suffer in that they have higher melt viscosity, poorer processing and poorer adhesion to some types of surfaces than propylene based copolymers. U.S. Pat. No. 5,118,762 addresses the industrial need for hot melt adhesives that have a low melt viscosity and high temperature resistance to shear. The solution in this document is the use of a predominantly branched styrene-isoprene-styrene (SIS) triblock copolymer with a tackifying resin that is compatible with the elastomeric isoprene block, e.g., diene-olefin copolymer resins, rosin esters or saturated petroleum resins, e.g., hydrogenated dicyclopentadiene resins such as ESCOREZ® 5000 series resins of the ExxonMobil Chemical Company.
Blends of isotactic polypropylene and ethylene propylene rubber are well known in the prior art, prior art Ziegler-Natta catalyst systems could only produce ethylene propylene rubber compositions with greater than 30% by weight ethylene at practical, economic polymerization conditions. There exists a need for polymeric materials which have advantageous processing characteristics while still providing suitable end properties to articles formed therefrom, e.g., tensile and impact strength. Copolymers and blends of polymers have been developed to try and meet the above needs. U.S. Pat. No. 3,882,197 to Fritz et al. describes blends of stereoregular propylene/alpha-olefin copolymers, stereoregular propylene, and ethylene copolymer rubbers. In U.S. Pat. No. 3,888,949 Chi-Kai Shih, assigned to E I DuPont, shows the synthesis of blend compositions containing isotactic polypropylene and copolymers of propylene and an alpha-olefin, containing between 6-20 carbon atoms, which have improved elongation and tensile strength over either the copolymer or isotactic polypropylene. Copolymers of propylene and alpha-olefin are described wherein the alpha-olefin is hexene, octene or dodecene. However, the copolymer is made with a heterogeneous titanium catalyst which makes copolymers which are non-uniform in compositional distribution and typically broad in molecular weight distribution. Compositional distribution is a property of copolymers where there exists statistically significant intermolecular or intramolecular difference in the composition of the polymer.
In U.S. Pat. No. 4,461,872, A. C. L. Su improved on the properties of the blends described in U.S. Pat. No. 3,888,949 by using another heterogeneous catalyst system. However, the properties and compositions of the copolymer with respect to either the nature and type of monomers (alpha-olefin containing 6-20 carbon atoms) or the blocky heterogeneous intra/inter molecular distribution of the alpha-olefin in the polymer have not been resolved since the catalysts used for these polymerization of propylene and alpha-olefin are expected to form copolymers which have statistically significant intermolecular and intramolecular compositional differences.
In two successive publications in the journal of Macromolecules, 1989, v 22, pages 3851-3866, J. W. Collette of E. I. DuPont has described blends of isotactic polypropylene and partially atactic polypropylene which have desirable tensile elongation properties. However, the partially atactic propylene has a broad molecular weight distribution as shown in FIG. 8 of the first publication. The partially atactic polypropylene is also composed of several fractions, which differ in the level of tacticity of the propylene units as shown by the differences in the solubility in different solvents. This is shown by the corresponding physical decomposition of the blend which is separated by extraction with different solvents to yield individual components of uniform solubility characteristics as shown in Table IV of the above publications.
In U.S. Pat. Nos. 3,853,969 and 3,378,606, E. G. Kontos discloses the formation of in situ blends of isotactic polypropylene and “stereo block” copolymers of propylene and another olefin of 2 to 12 carbon atoms, including ethylene and hexene. The copolymers of this invention are necessarily heterogeneous in intermolecular and intramolecular composition distribution. This is demonstrated by the synthesis procedures of these copolymers which involve sequential injection of monomer mixtures of different compositions to synthesize polymeric portions of analogously different compositions. In addition, FIG. 1 of both patents shows that the “stereo block” character, which is intra or intermolecular compositional differences in the context of the description of the present invention, is essential to the benefit of the tensile and elongation properties of the blend. In situ blends of isotactic polypropylene and compositionally uniform random ethylene propylene copolymers have poor properties.
Amorphous polyolefins, such as atactic polypropylene, have no crystalline network and thus have poor cohesive strength. To improve cohesive strength high molecular weight amorphous polyolefins are needed in high concentrations and this leads to high viscosity and poor processability.
Moreover, all of these compositions either do not meet all of the desired properties for various applications, and/or involve costly and burdensome process steps to achieve the desired results. It is also desirable in packaging to have adhesive compositions that have suitably low melt viscosity for high speed automated coating processes, a sufficiently long time before hardening to preserve sufficient adhesion (known in industry as “open time”) in assembly operations such as box closures, yet a quick enough setting speed to allow shortest time application of adhering pressure (known in industry as “setting time”).
As set forth in greater detail below, certain aspects of this invention relate to the use of a peroxide or other free-radical initiator to provide a modified polymer or polymer blend. The use of peroxide to degrade certain polymers has been published in the literature.
For example, peroxide-initiated degradation of certain polypropylene resins is discussed generally in the article by Tzoganakis, et al., entitled “Production of Controlled-Rheology Polypropylene Resins by Peroxide Promoted Degradation During Extrusion,” pp. 170-180, Polymer Engineering and Science, Vol. 28, No. 3 (1988), and in the article by Rosales, et al, entitled “Viscoelastic Behavior of Controlled-Rheology Polypropylene Resins,” pp. 153-169, Materials Engineering, Vol. 4, No. 2 (1993).
Gahleitner et al., U.S. Pat. No. 5,705,568, relates to chemically degraded block copolymers. That patent discusses the use of peroxides to degrade elastic polypropylene homopolymers and copolymers with stereoregular block arrangements, blocks of isotactic and atactic propylene sequences. The patent discloses using minor amounts of peroxide, from 0.001 to 0.8% by weight, preferably 0.05% to 0.5% by weight to raise the melt flow index (MFI). The patent also discloses the addition of fillers, stabilizers and mould release agents. However, this patent does not discuss adhesive compositions.
Peroxides have also been used to oxidatively degrade olefinic polymers and/or polymer blends useful as lubricant compositions, as discussed, for example, in Gordon et al., U.S. Pat. No. 4,743,391 and Chung et al., U.S. Pat. No. 4,749,505.
Peroxides have also been used as curing agents in elastomeric blends. Duncan, U.S. Pat. No. 4,143,099, for example, discusses the use of peroxides in curing and “semi-curing” elastomeric blends, by adding the peroxide curing agent while masticating and shearing the mixture of polymers, and completing the semi-curing of the polymers before the onset of melting.
The present invention is directed in general to providing improved adhesive compositions, and processes or methods for making such compositions.